In wells and the like for collecting buried resources such as petroleum and natural gases, a slurry for civil engineering and construction typified by a drilling mud and a drilling cement slurry has been conventionally used.
The drilling mud plays roles in, for example: transporting drilled clasts, drilling wastes and the like; improving lubricating properties of bits and drill pipes; filling in holes on the porous ground; balancing out the reservoir pressure that results from the hydrostatic pressure (pressure from the rock stratum); and the like. In general, the drilling mud contains water and bentonite as principal components to which barites, salts, clays and the like are further added, whereby intended performances can be achieved. Such a drilling mud is demanded to have appropriate flow characteristics such as having temperature stability, not being significantly affected by variation of concentrations of electrolytes (for example, carboxylic acid salts) in the ground, and the like. To meet such demands, adjusting the viscosity of the drilling mud, and inhibiting dissipation of the moisture contained in the drilling mud (hereinafter, may be also referred to as “dehydration”) may be required. For adjusting the viscosity of the drilling mud and for inhibiting the dehydration, a method which includes adding a polymer, for example, starch, a starch ether (carboxymethyl starch, etc.), carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose or the like is usually adopted.
However, the addition of such polymers may extremely elevate the viscosity of the drilling mud, whereby injection of the drilling mud by a pump may be difficult. Moreover, there may exist disadvantages that dehydration of starches and derivatives thereof may not be sufficiently inhibited within a temperature range exceeding about 120° C., and the dehydration may not be sufficiently inhibited by carboxymethyl cellulose and carboxymethyl hydroxyethyl cellulose within a temperature range of 140° C. to 150° C.
On the other hand, the drilling cement slurry is used for, e.g., fixing the casing pipe in the well and protecting the inside wall of the well through cementing which comprises injecting the drilling cement slurry into tubular void portions between the stratum and a casing pipe installed in the well, followed by hardening therein. In general, the injection of the drilling cement slurry into tubular void portions is carried out by using a pump. Thus, the drilling cement slurry is required to have extremely low viscosity and not to be accompanied by segregation such that the injection thereof by using the pump can be readily carried out.
However, in cementing a well, a defect is likely to occur in a cemented part due to: material segregation; dissipation of the moisture to cracks in the well; and the like. Accordingly, a dehydration-reducing agent such as walnut shells, cotton seeds, clay minerals, polymer compounds and the like has been employed to be added to the drilling cement slurry, and in particular, vinyl alcohol polymers being a polymer compound are a well-known dehydration-reducing agent.
In regard to the vinyl alcohol polymer as a dehydration-reducing agent, for example, Patent Document 1 discloses a method in which a vinyl alcohol polymer having a degree of saponification of at least 95 mol % is used; Patent Document 2 discloses a method in which a vinyl alcohol polymer having a degree of saponification of 92 mol % or less is used; and Patent Document 3 discloses a method in which a vinyl alcohol polymer having a degree of saponification of at least 99 mol % is used. However, according to these methods, performances of the dehydration-reducing agent at high temperatures in particular, may be insufficient, or feeding of the drilling cement slurry by the pump may be difficult through elevating the viscosity.
In order to moderate the viscosity elevation of the drilling cement slurry and/or to improve deteriorated performances of the dehydration-reducing agent at high temperatures: Patent Document 4 discloses a method in which a vinyl alcohol polymer crosslinked by melamine-formaldehyde is used; Patent Document 5 discloses a method in which a vinyl alcohol polymer produced by crosslinking of hydroxyl groups of a vinyl alcohol polymer by an acetalization reaction or the like is used; and Patent Document 6 discloses a method in which a vinyl alcohol polymer produced by pH-sensitive crosslinking using a boron ion, etc., is used. Although a given effect of improving the performances of the dehydration-reducing agent at high temperatures is achieved according to these methods, it is necessary to carry out the reaction with the crosslinking agent after a water soluble vinyl alcohol polymer is produced beforehand, and thus the cost is likely to be increased.